I use a fine & medium gravel in my reverse flow UGF. The fine gravel is about 1/16" or larger, and the medium gravel is about 3 times that size. Just big enough not to pass through the grid plate.
BTW, I am with RTR on the RFUGFs. I have had great success with mine.
BTW, I am with RTR on the RFUGFs. I have had great success with mine.